Cerebral blood flow (CBF), cerebral blood volume (CBV), the CBF/CBV ratio – an index of the hemodynamic reserve capacity – the rate of oxygen metabolism (CMRO2), and the fractional extraction of oxygen by the brain (OEF) were studied by positron emission tomography (PET) in the cortical territory of both internal carotid arteries in 15 cases of transiently symptomatic or progressive extracranial atherosclerotic carotid disease. None of the patients had a major stroke or had a significant neurological deficit except 1 whose damaged hemisphere is excluded from study. All were asymptomatic at the time of PET scanning. Values were obtained in the middle cerebral artery (MCA) distribution, and in the anterior and posterior borderzone regions. Eight cases had unilateral carotid stenosis of 80% or greater and 7 had unilateral or bilateral occlusion of the origin of the internal carotid artery. Results obtained in patients were compared using Student's t-test, to those obtained in neurologically normal, elderly volunteers. Patients with carotid stenosis had a significantly decreased CBF (p<.025) and CBF/CBV ratio (p<.025) selectively in the anterior borderzone regions. This was accompanied by a trend toward elevated OEF and declining CMRO2 values. Patients with carotid occlusion had significantly decreased CBF (p<.005), decreased CBF/CBV ratio (p<.005) and decreased CMRO2 (p<.025) in the ipsilateral anterior borderzone and MCA territories. Similar changes were present in the opposite hemisphere of patients with bilateral carotid disease. These results indicate that carotid stenosis is associated with hypoperfusion and diminished hemodynamic reserve capacity in the anterior borderzone, and that carotid occlusion produces more widespread hypoperfusion and metabolic depression.